Personal Notes

SNM 96

Denver, Colorado

1 June 1996 to 6 June 1996

by Robert E. Zimmerman

Saturday

American College of Radiology Nuclear Medicine Accreditation Committee

I'll bet that many of you did not realize that nuclear medicine clinics
may go the way of mammography clinics. Yep! Accreditation may be just around
the corner. ACR is planning for it, working on the criteria, etc. There
is still several years before you will hear anything significant about
this. But you will hear more.

Nuclear Medicine PACS

A small group of nuclear medicine PACs enthusiasts met with the encouragement
of Nicole Ranger from Pittsburgh. Nicole is working hard on pulling material
together on all aspects of PACS for the nuclear medicine community. The
major goal is to educate the consumers (nuclear medicine professionals)
and manufactures on the needs and specifics of nuclear medicine PACS. This
group will be a focus group of the Computer & Instrumentation Council
and a task group of the American Association of Physicists in Medicine
(AAPM).

Sunday

Sunday turned out to be a perfect day to visit the Rocky Mountain National
Park. We were lucky to have a very experienced native guide on our quick
visit. There was still lots of snow and wildlife, right at roadside. Our
hike was blessed with plenty of sun, some snow and beautiful mountain lakes
and streams. If ever in the Denver area make time to visit this beautiful
park.

ABSNM held a board meeting Sunday evening. The new president is Dr.
Ahluwalia who takes over from Dr. George Zubal. George worked hard on the
revitalization of the ABSNM during his tenure. The organization is trying
to make itself more attractive to nuclear medicine scientists i.e. physicists,
pharmacists, health physicists and others who work in the physical and
chemical side of the nuclear medicine profession. It has become very clear
to me that certification of all professionals will be increasingly necessary
for many of us as we continue to work in nuclear medicine. ABSNM provides
a very reasonable option for this certification.

ABSNM also held 20th anniversary reception buffet for all diplomates
and friends on Monday evening. Congratulations for 20 years of service
to nuclear medicine professionals.

Monday

There was several themes that ran through this meeting. Dominant in
my view was the impact of coincidence detection with SPECT cameras. Not
more than one unit had been sold, installed and put into regular use but
it has inspired numerous imitators. ADAC introduced the modern version
about 18 months ago. Most companies are talking about it and trying to
come up with a marketing campaigns. At least one company is actively NOT
talking about it. (Park can do it better with coded apertures, see below.)
Another theme is the wide role F-18 (as FDG) seems to have in the future
of nuclear medicine. How many of us can actually do this back home? Well,
maybe soon. Attenuation with scatter correction and ordered subsets were
much talked about throughout the physics sessions.

The move to special meetings (breakfasts, receptions, dinners) for physicists
is interesting. ADAC started this about 2 years ago. This year Siemens
had a breakfast, Park had an evening supper and ADAC continues with an
evening buffet. There may have been others. This is in addition to the
user meeting, which happened on Sunday, this year, I think. I was in the
Rocky Mountain Park. Most sensible thing I did that week (see above). It
is flattering for the physicists to be invited to hear all this information
in more detail than you could learn from the exhibit floor. But it will
obviously be impossible to do it all. Let me make a proposal to all the
manufacturers. Widely advertise your website. Put in-depth information
on the website. Provide for a BBS-like Q and A dialog to be also on the
website, something like Lunis. Let your technical people participate in
the dialog with the users. Then let us have the early mornings and evenings
free at SNM and RSNA. You get your story out to the physicist and we can
enjoy the meeting and not feel guilty about missing your latest information,
'cause all we have to do is look at your website.

Plenary Session

As I age I find that I spend more time at these plenary sessions. Many
times I know the speakers, know the awardees and understand the work for
which they are receiving the awards. It makes the session more interesting.
I used to think they were something of a distraction from the main part
of the meeting. Interesting how we change over the years.

Theme of the meeting was Nuclear Medicine: 100 years in the making.
The theme is inspired by Henri Becquerel's discovery of radioactivity in
1896. Henry Wagner reviewed the history of nuclear medicine from that point
in time to the present. Dr Louis Sokoloff , of NIMH, reviewed the history
and development of functional brain imaging in the featured plenary session
talk. Starting with the 1948 groundbreaking work of Kety with nitrous oxide
through recent work with FDG was presented by one of the history makers.

Tom Budinger, well known to physicists all over the nuclear medicine
world, was awarded the Hevesy award in honor of his work at the Donner
Labs, Berkeley. Dr Tom Miller presented the award and reviewed Budinger's
long, productive scientific career.

The Abersold award went to Kenneth Krohn a chemist at University of
California for his contributions to F-18 and Tc-99m chemistry. The award
was presented by Michael Welch of St. Louis.

The Cassen award. was presented by Abass Alavi to David Kuhl in recognition
of his early work in developing tomographic instrumentation, starting in
the 1960s, even before there was computed tomography.

Scientific Sessions (and other meetings)

Did you ever wish that the SNM would make a machine readable version
of the abstracts available. In preparing this I certainly could have used
it. Just for the titles and authors. It would have been very useful. I
suggest putting it on the SNM website next year.

FDG imaging seems to be taking the world of (SPECT) oncology by storm.
Everyone want to do it, and cheaply. Just how useful the collimated approach
is going to be is still an open question in my mind. More below.

There was a lot at this meeting concerning the imaging of F -18 on hybrid
cameras. ADAC has moved to a 5/8" crystal and faster electronics.
It is out of alpha trials and into beta trials as I write. No other company
is as far along in the development process. Siemens is talking about, in
2-3 years, a new scintillator block detector doing hybrid studies. It would
consist of YSO sandwiched to LSO. See below.

Attenuation correction is still receiving a lot of attention but with
increased emphasis on scatter corrections and resolution recovery. As has
been stated in several places recently, attenuation correction is no panacea.
It does not suddenly make diagnosis of CAD more accurate. In my opinion,
neither will scatter correction or resolution recovery. There is still
the INTERPRETATION of the images. If the physician has not been trained
to interpret the new, improved images there is no way diagnosis will be
improved. If incorporating the improvements into your practice, you must
go through the necessary self-training to understand the new images. For
that reason those correction methods which allow for the preservation of
the uncorrected, familiar image should have a real edge at this early stage.

SPRINT-II, with its slit collimators providing magnification, can be
useful for imaging small animals. This penetration becomes a problem when
the isotope is F-18. The authors describe considerable success by taking
a scan with the slits blocked by U to estimate, and correct, the penetration
component. They propose that it could be used in more general imaging situations,
also.

Can scattered photons help us? This paper reported efforts at reducing
noise when performing scatter correction. In their simulation they found
that including counts below 119 Kev increases noise in reconstructed images.

This was the clear winner in the competition. Extremely well presented,
addressed a significant problem with impressive results. In order to put
a 3D brain image into the Talairach Proportional Coordinate System it is
necessary to find the mid-sagittal plane. The automated method here can
find the plane in SPECT, PET and MRI scans rapidly and with high accuracy
and precision.

The authors extended their previously reported technique for hearts
to brain imaging. They found that electrical stimulation increased ECD
amounts by about 20% for local regions of the brain. Registration with
MRI provided anatomical reference.

Using simulation studies, these authors found that the acquisition parameters
strongly affected the bias in the estimate of kinetic parameters. High
acquisition rates caused bias in the kinetic parameters because of increased
noise. In certain cases longer acquisition rates led to reduced bias along
with reduced storage requirements.

Session 11: Instrumentation and Data Analysis: SPECT - Attenuation

It is generally accepted (believed, hoped) that when attenuation, scatter
and resolution corrections are applied to cardiac SPECT more accurate,
and hence more useful images will result. One of the main problems is to
get all these corrections done accurately, with no noise or artifacts and
do it all quickly. It has been tough. At this meeting quite a few papers
addressed this issue. Many of them were in this session.

Using the Cramer-Rao bound for several parameter estimation tasks, the
authors found that an exact attenuation correction method recently reported
by Metz is significantly better than previously reported techniques (Bellini,
Gullberg, Treitak & Metz). Furthermore they confirm that this new method
is nearly optimum and there is little expectation for significantly better
methods (better in the sense of increased estimation performance) to be
developed.

The method described in this paper is valid for only the heart, not
surrounding tissues. This is characteristic of several current methods.
Scatter corrections are not addressed. Bellini attenuation correction was
performed and detector response is compensated for using the frequency
distance principle in an iterative restoration technique in projection
space. Reconstruction is by Fourier backprojection. It was found that more
accurate bulls-eye plots and better noise properties resulted form the
studied algorithm

Can an analytic attenuation correction method approach the ML-EM iterative
reconstruction results? These authors found, in a simulation study, that
the Bellini attenuation correction method and Fourier backprojection reconstruction
with a 3-head, fan beam collimated system using a line source, can produce
bulls-eye polar maps comparable to the ML-EM algorithm. The truncation
in the attenuation map only slightly degraded performance.

Can the emission data alone provide enough information for attenuation
correction? The authors found, with real experimental data, that in brain
SPECT corrections worked pretty well but that for heart SPECT corrections
were a problem.

The authors compared F-18 attenuation correction on a real PET system
with attenuation correction on a Picker STEP system. They found that the
PET system could determine the attenuation map with an accuracy of about
5% while the accuracy of STEP was only 20%. This may be due to scatter.
They found accuracy improved with reduced pixel size and indicated 128
x 128 matrices were preferred for STEP.

The authors discussed a means of scatter correction for the transmission
data from scanning line source measurements performed according to the
method of Bailey and Hutton. They found that the correction for Tc-99m
scatter into the Gd-153 window substantially improved the attenuation correction
results.

Three optimized scatter correction methods were evaluated, using a maximum-likelihood
procedure, with regard to performance in activity quantitation and lesion
detection tasks. The optimum k subtraction factor for the Compton window
subtraction method was determined. The widths of the energy windows were
optimized in the TEW method. Optimized weights for a generalized spectral
fitting method from 92-160 keV were obtained. It was found that overall
the best performance was obtained with the spectral fitting method having
bias < 4% and the lowest variance. TEW had bias of <7% but higher
noise than the Compton window method, which had bias ranging from -2 to
22%. The author concluded by questioning the value of scatter correction
for these tasks.

The authors investigated the effect of scatter correction on signal
detection. Real observers evaluated simulated lesions in a simulated liver
background. The dual photopeak window (DPW) scatter correction method was
used. ROC curves were derived from the observer scores. No significant
difference in the ROC curves for the primary photon images and the scatter
corrected images was found. The authors concluded that improved quantitation
does not lead to improved detection.

Six energy windows were used, three for Tl-201 and three for the Tc-99m
transmission source. The TEW correction method was used for scatter and
spilldown correction. The transmission source was an "area" source
composed Tc-99m filled tubing. The method was reported to be practical
and effective.

The authors tackled the often repeated story that attenuation correction
causes spurious increase in counts in cardiac regions. They studied scatter
and attenuation with simulation methods and applied two scatter correction
methods: a deconvolution method and a model-based approach. They found
that both methods improved the spurious count problem with the model-based
approach being better but costing more computer time.

The authors extend previous work in Slab Derived Scatter Estimation
with parallel hole collimators to fan beam collimators. They found that
the technique has some problems when used with inhomogeneous media and
Tl-201. The results for Tc-99m were better. More importantly, for me, they
concluded that it is not enough to just do attenuation correction. Scatter
correction must also be done.

DISA means dual isotope, single acquisition. There is concern that the
spilldown from F-18 will cause diagnostic errors in the MIBI interpretation.
A third energy window at 170 keV +/- 10% was used to estimate this spilldown.
The subtraction factor was 0.67. Phantom measurements indicate the errors
were reduced. In patients the images looked better but no diagnostic improvement
could be detected.

Instrumentation Council Business Meeting:

Steve Bachrach called our attention to the new interchange format for
nuclear cardiology based on CD-ROMs. It is based on the DICOM file format
and allows images from any modality to be viewed with a universal reader.
He was passing out free CDs with samples. Manufactures now need to support
it. Steve ran a full session on this later in the week to explain more
fully what this does for nuclear medicine.

Nicole Ranger asked that a PACS Focus group be formed. It was. See above
for what this group is trying to do.

Another focus group on telemedicine headed by Trevor Cradduck will be
writing a final draft this summer.

The 1997 mid-winter meeting will be in Palm Springs. Fred Fahey may
be running this meeting. Contact him with your ideas. Jack Correia may
also be working on this. Contact him, too, if you have any topics that
can attract physicians and physical scientists to Palm Springs.

The SNM program committee is entertaining ideas for new presentation
methods at the annual meeting. For example some type of interactive presentation
may be tried in the future. How about those walking poster sessions? Do
you like them? Let them know. See below for my comments on them.

Session 34: Instrumentation and Data Analysis: General 1

If mammoscintigraphy is ever to prove useful special purpose cameras
will have to be developed specifically to image the breast with the highest
possible spatial resolution consistent with useable sensitivity. The UCLA
group reported their progress with a CsI scintillator with a silicon photodiode
readout which they hope will provide intrinsic resolution of <2 mm.
They admit that photodiode noise will be a major problem.

Dr Osipova reported on their Monte Carlo simulation of pinhole imaging
on the multiwire proportional camera. With a magnification of more than
10, two mm in vivo resolution is predicted and they feel that useful mouse
heart images will be obtained.

The Arizona group updated us on the FASTPECT - semiconductor version.
By summing adjacent 125 um pixels in the 48x48 array 7.5% energy resolution
was obtained with a spatial resolution of 214 um at 140 keV. Next readout
will consist of 64x64 array with 380 um pixels.

This is the institution that has the ADAC prototype SPECT/PET camera.
The current prototype has crystal thickness of 5/8" and improved electronics
to achieve much higher count rates than the original prototype (see below).
Dr Glass reported on his comparison of the prototype camera to a real PET
system. He feels that the thicker crystal and improved electronics have
lead to a machine that is significantly better than the original and is
useful for oncology, brain and heart imaging.

ADAC Physics Reception:

Attenuation Correction

ADAC presented technical details on their latest products at an evening
buffet. The ADAC collaborators presented some details of their ongoing
clinical trials. Mark Groch of Northwestern told us about his experiences
with the attenuation correction - present generation. While, in general,
it worked pretty well he did notice some errors due to downscatter of Tc
photons into the Gd window. Jim Cullom, Emory U. told us about improvements
he was working on that would provide scatter correction, downscatter correction
and resolution recovery. He uses a ML reconstruction method.

SPECT/PET Camera

Gerd Muehllener reported in considerable detail regarding the latest
PET coincidence camera improvements. The crystal thickness is 5/8"
an the counting rate capability has been improved to 2,500,000 cps max.
singles rate. The information was presented:

Resolution in coinc

4.7 mm

Sensitivity

100 kcps/uCi/cc

Singles deadtime

5% at 1 Mcps

Max singles rate

>2.4 Mcps/det

Crystal Thick

10 mm

16 mm

At 511 keV:

Intrinsic Sens

28%

45%

Coinc Sens

7.8%

20.2%

Coinc Frac

0.8%

2.0%

Singles Mode:

Intrinsic

3.5 mm

3.9 mm

System Res

7.05 mm

7.25 mm

Dr Glass again told us about his promising clinical results.

The results are impressive and ADAC is clearly leading this race to
get coincidence imaging working on a gamma camera. But I hope we can hear
that it is routinely available soon. It is getting to be a long wait.

Including the scatter psf in iterative fully 3D ML-EM has been shown
to work better than subtraction based scatter correction. The problem is
the tremendous amount of computation. By using separate projection and
backprojection matrices with ordered subsets EM speed increases of 170
were realized.

These investigators reported a factor of 10 reduction in computation
time with ordered subsets-ML vs ML-EM when doing collimator blur correction
with attenuation correction for circular orbits. Elliptical orbits take
two or more ML-OS iterations to reach the ML-EM solutions. This was a noise
free simulation study.

These authors reported on a way to efficiently incorporate blur and
attenuation correction using a Gaussian diffusion method. This is an approximate
method that seems to agree well with the more complete and detailed convolution
filtering method.

Session 51: Instrumentation and Data Analysis: General II

M. Singh has worked extensively with Compton scatter gamma camera using
a Ge detector as the first stage and a NaI camera as the second stage.
These authors have extensively studied Compton scatter cameras using a
Monte Carlo simulation and conclude that having a Si camera as the first
stage and a CdZnTe as the second stage offers potential performance advantages
particularly at higher energies.

The authors attempted to implement the dual photopeak window (DPW )
scatter correction method on a dedicated brain camera (Ceraspect). A stable,
unique calibration curve could not be obtained. Simulation of the calibration
process confirmed the difficulties. The DPW method may not have sufficient
stability to be practical, especially for a brain system.

Pinhole collimator! Simulation and phantom studies confirm that resolution
of about 4 mm and sensitivity of about 2x the HR collimator can be obtained
at the center of the breast. Modifications to a camera could permit compression
and biopsy. Encouraging!

The authors were interested in quantitatively imaging therapy doses
of I-131 labeled monoclonal antibodies with multiple head gamma cameras
and were concerned about deadtime in the dual window mode. They performed
experiments on a Picker 3-head system and on a Siemens MS-2. The paralyzable
model was used. For less than 30 mCi in the field they found that accurate
dead time corrections could be performed and that the Siemens was a considerably
faster camera.

A 1.2 cm diameter beta ray camera using CaF2(Eu) is used to detect tumors
emitting positrons or electrons. There is a problem with annihilation photons
when betas are used. Rejection methods using beta-gamma coincidence techniques
were studied and reported. The background can be reduced using these techniques.

Bi-213 emits alphas and a 440 keV gamma ray. In order to image the 440
keV gamma ray with HEHR collimators the gain of the gamma camera had to
be adjusted. After adjustment the Bi-213 could be imaged successfully.

I missed the presentation of this paper but the abstract was so interesting
I include it in this summary. Authors had 55 patients, 20 of whom had cath.
The instrument was a GE L-shaped Optima with scanning Gd-153 line sources.
Sequential TCT data acquisition followed the stress study and 4 hours later
the redistribution study. There were 3 reconstructions done. Sensitivity
and specificity are given below:

Reconstruction Method

Sens

Spec

FBP w/o compensation

83

58

ML-EM w/ AC

80

72

ML-EM w/ AC & SC

88

67

Clearly something is happening but is it all good? Does it need resolution
recovery?

In a study with 21 patients using a fan beam collimator with a 3-head
camera system the investigators found significant differences between corrected
and uncorrected using the ML-EM algorithm. Some of these differences were
attributed to scatter. They concluded that BOTH image sets are needed to
make an accurate diagnosis.

Although the spatial resolution correction was done with the frequency
distance principle the noise problems seen by others were not observed.
While admitting the numbers are too low, Cullom is encouraged to explore
his more complete correction method.

Presented by Berman, before No. 314 because of scheduling problems.
Some people think that prone imaging obviates the need for attenuation
correction by decreasing diaphragmatic attenuation. In this study with
20 low-likelihood patients they found near perfect agreement between supine
images with attenuation correction and prone imaging with no correction.
Maybe we only have to roll the patient over and stop all this effort. In
defense of more physics work, there was no correction for Tc spilldown
to Gd window and with stress imaging there is low liver uptake. So the
results are not conclusive.

This Ann Arbor group has a Picker 3-head camera with STEP hardware but
only use only one fan beam collimator and 2 parallel hole collimators.
This permits them to acquire datasets and calculate straight uncorrected
or corrected heart scans. They used this setup to test 180 vs 360 rotations.
The line source was Am-241. They found that there was little difference
between 180 and 360 with or without attenuation correction AS LONG AS PROPER
DATABASES WERE USED. I guess it is obvious that we need to understand and
have a knowledge base to make decisions.

Using a dedicated first pass camera, this group found a quantitative
first pass study helped to distinguish scar from inferior wall attenuation
in 15-30% of patients that were known to have fixed inferior wall defects
from previous SPECT test. Does first pass test have a role to play in nuclear
cardiology?

Thursday

Session 79: Instrumentation and Data Analysis: SPECT

The abstract talks about hand waving claims for an LSO detector. The
presentation was about a YSO/LSO phoswitch detector that the authors hope
will prove capable of SPECT and PET imaging. The simulations were not particularly
convincing.

This study evaluated the Cramer-Rao bound for a Compton scatter camera.
The model used was a simplified version in order to make the calculation
tractable. I did not really understand the conclusion. It sounded like
the authors were encouraged enough to continue the study with a more realistic
and complex model.

This simulation study seemed to come to a positive conclusion on the
possibility of pinhole brain SPECT imaging with 511 keV photons. In poster
No. 965 this same group presented experimental data on I-131 SPECT imaging
of the brain and it seemed to work, too. Compensation for those penetrating
high energy photons is important.

This is a very clever idea. An array of 8-10 line sources of 26 cm length
are mounted opposite the detector. Line spacing is optimized in simulation
study. Requires 360 rotation. Can do simultaneous emission-transmission
scanning. Lower cost as hottest central sources can be moved to outer positions
as they decay. They will be studying the effect of scatter. I think this
is the idea that Siemens has bought into for their new e.cam system.

Walking Posters

Several sessions called walking posters were held. These require the
audience (smaller than in a meeting room) to walk from poster to poster
and listen to the presenter. It is hard to sort of stand in one spot for
90 minutes, it is hard to hear if the crowd exceeds 20 or so. It is nice
to interact with the presenter and interested participants. On balance
I do not like them, however. A more standard poster session where you can
circulate and speak to a select group of authors of your choosing is better,
I think.

The authors extended their work for activity estimation with restoration
for stationary psf to the case where the psf is depth dependent. This represents
the case in SPECT. Restoration was performed using a Metz filter that was
depth dependent. The recovery coefficient increased to about 1. There was
a some increase in noise. They conclude that nonstationary resolution recovery
is necessary for ROI activity estimation.

This physical phantom has fillable organs with MIBI uptake (includes
R and L myocardium and chambers, R and L Styrofoam beads for lungs, liver,
gallbladder and bowel. It has removable breasts. Imaging tests were close
to those of real humans.

Authors found little computational cost for including simultaneous reconstruction
of the attenuation corrected low and high photons of Tl-201. They found
significant improvement in the when using two photons. Of course, ordered
subsets were used.

The authors emphasized the importance of high energy resolution and
an accurate scatter model. Scatter correction techniques always came out
second best to a good model calculation. At low count rates it is better
to use the scatter than not use it.

Other posters seen:

Numerous posters on FDG imaging in oncology were up. I could not find
any negative results. It seems that FDG is a real good thing and we all
should begin imaging it, even if only poorly, like with a big collimator,
maybe. I searched in vain for guidance on just how small the lesion could
be before they could be visualized. In other words what is the threshold
for visibility in PET and collimated FDG? I could not find the answer to
this.

Fe-52 is a beta emitter. ECAT-ART is a real PET system but only a partial
ring. Sensitivity of ECAT-ART is 11400 with resolution of 6.0 to 7.7 mm.
The MS-2 sensitivity was 70 and resolution was 12.7 to 17.9 mm. Both instruments
were able to localize extramedullary erythropoiesis but EXACT-ART could
localize activity within the bone marrow.

Using their own technique which uses a single fan beam collimator with
two parallel hole collimators, the authors claimed that attenuation correction
improve identification of defect reversibility. There were no negative
comments with regard to attenuation correction.

These authors found that in a group of 70 patients viewing the rotating
projections lowered sensitivity, but raised specificity and accuracy with
or without attenuation correction. The best results seemed to be with attenuation
correction, but it was difficult to be sure.

Looks like the 70 patients from the above study are back again. This
time it was found that ADAC's Vantage attenuation correction increases
normalcy rate. Image quality was rated better for attenuation corrected
images. Does not seem to be any problems and AC helps.

Adding two extra windows one FWHM below and above the photopeak centerline
allows scatter (and spilldown) information to be collected. This is something
like a TEW correction. A solid phantom containing a Tl-201 line source
and Co-57 transmission source was scanned to determine if it worked. It
did.

The dual head Picker 2000 now has the standard scanning line source.
Scatter correction and crosstalk is included. No surprises. It works pretty
good.

No 956 THE EFFECT OF DEFECT SIZE, LOCATION, AND CONTRAST ON THE DIAGNOSIS
OF MYOCARDIAL DEFECTS IN SPECT IMAGES WITH AND WITHOUT ATTENUATION COMPENSATION
KJ LaCroix, BMW Tsui UNC, Chapel Hill

This was an attempt to address the question of how defect size, location
and contrast may affect the diagnosis of myocardial defects with and without
attenuation correction. It was a simulated study using noise free projections
of the MCAT phantom. A computer did the diagnosis. Location is the big
factor and attenuation correction helped in areas subject to attenuation.
Not surprising results, is it?

Providing depth dependent resolution recovery takes lots of computer
time. There were several papers that addressed this problem. In this work
resolution recovery was performed using a modified depth-dependent Wiener
filter. Authors report good convergence properties and accurate results
with this method.

CT was registered with SPECT in the abdomen and thorax. Errors in x
or y of 5 mm led to quantitation errors of <10% . Quantitation errors
could be much higher at the edges of an organ. Errors in z of 10 mm (CT
slice thickness) could lead to very large errors depending on size of object.

We often forget or ignore details, like: does the isotope stay in solution
within our phantom? These authors found significant problems with uniform
distributions of In-111 until they used DTPA to chelate the In-111. We
need more hints like these compiled someplace.

This is an attempt to improve on the STEP system that Picker supplies
with the Prism 3000. STEP was the first of the commercial releases of cardiac
attenuation correction. Picker is to be congratulated for making it available.
But it is not the final solution. There are problems with the commercial
version of STEP that we, and others have seen. Problem 1: there is no way
to ease into attenuation correction. You have to stop using the parallel
hole collimators, install 3 fan beam collimators and start using the iterative
reconstruction program. At this point you can compare non AC images with
AC images. But you have no diagnostic skills developed to use either the
fan beam images with or without AC. Problem 2: there is no scatter correction
in the commercial version of STEP. There is downscatter correction. Problem
3: There is no depth dependent resolution recovery in commercial STEP.

What is needed is a way to ease into attenuation correction by retaining
the original parallel hole images, allowing comparison of the old with
the new, improved "fully" corrected images. Several groups have
been working on such "improvements" to STEP. The U Mass work
is reported in this abstract. I also learned at this meeting that a group
associated with EP Ficaro from U Michigan has a similar approach.

The U Mass approach uses one fan beam collimator and a line source from
the original STEP hardware and uses two parallel hole collimators. The
transmission study is acquired after the emission study. TEW scatter correction
is provided. Reconstruction is by FBP for the old and familiar AND by ML-EM
for the attenuation corrected images. No resolution recovery operation
is described. Some artifacts remain in the corrected images but "physician
education" can be done by using the old images to learn how to use
the "new, improved" images.

The authors found that the wall thickness of the lesion inserts gave
significant errors with regard to nominal lesion size. This led to more
favorable object contrast than expected and overestimation of system performance.

A stochastic ray tracing program that explicitly models hole shape,
fan beam or parallel hole parameters, resolution vs depth etc. is described.
Optimum collimator for brain SPECT is arrived at by using a MRI brain scan
with radiopharmaceutical distributions as a source object and comparing
reconstructed image with object. Sound like a good way to design a collimator.

The authors incorporated two windows on either side of the Tl-201 photopeak
ala TEW, but not abutting the photopeak window. Rest of the method seems
like standard STEP. The images were better with scatter correction.

I have heard that Toshiba has patented TEW and is the only manufacturer
that can "sell" TEW so I guess the other companies have to come
up with workarounds. Isn't that like patenting the photopeak window? I
do not think that it is reasonable to patent acquisition windows or what
you can do with the data from those windows. Hope we are not breaking the
law. I would hate for the Japanese industrial machine to come down on Harvard.

In theory one can obtain the regional myocardial uptake per gram of
tissue of a tracer in the myocardium. In PET it is easier that in SPECT.
The authors extend their work in PET partial volume correction to SPECT
in this paper. Method involves two simultaneous emission and transmission
acquisitions. One for blood pool and one for perfusion. Attenuation and
scatter correction are performed. The estimate of uptake can be made independent
of partial volume if emission and transmission images have same partial
volume errors. Sounds futuristic. We can't even get attenuation correction
to work where I come from.

The details must be published elsewhere. The method uses two convolution
equations applied in frequency space and incorporates restoration filtering.
Makes contrast better. No statement as to how noise changes.

The details of the method were not easy to discern. Water equivalent
depths were used to account for the non-uniform attenuation in the chest.
Since it uses the psf I would think that it would perform some sort of
resolution recovery also, but that was not discussed in this poster.

GE has apparently relegated the development of coincidence imaging to
the academic arena. I suppose that this is some kind of holding pattern
so they can determine if a first class, expensive development is needed
to catch up to ADAC's position. Meanwhile it gives the students at U Washington
some very interesting projects.

But I think we would all agree that a PET/SPECT camera has to be designed
from the ground up to be suitable for PET. It is hard to do it good enough
to be clinically useful.

Scientific Exhibits

I did not have any time to spend at the scientific exhibits bit I do
want to call attention to one:

The Joint Program in Nuclear Medicine is in a position to test many
different vendors cameras. Even to compare (eventually) collimated FDG
imaging with PET in certain circumstances. This web project: http://www.med.harvard.edu/JPNM/physics/511keV/511keV.html
is a compilation of our results and notes to date. We intend to make this
useful to the full nuclear medicine community. We welcome your comments.

Commercial exhibits

3M Medical Imaging Systems

The business of the dry laser printer started by Polaroid has serious
competition. 3M Medical Imaging Systems, St Paul, MN, has one. Claims 1024
gray levels and 300 dpi. I wonder if anyone has ever compared these things?
I have always thought that these things would be replaced by CRTs, but
I seem to be wrong.

ADAC

ADAC continues to lead in coincidence counting, announcing 5/8"
crystals and new electronics that push max. singles rate to 2.4 Mcps. Their
Vantage attenuation correction seems to want downscatter correction. They
have a website and it seems to have some good information on it. They may
be incorporating net browser ideas into their imaging viewing programs.
(See report above on their physicist meeting for more ADAC details.)

Digirad Corporation, San Diego CA

Bill Ashburn was showing a semiconductor camera (some kind of CdZnTe,
I think). It was one quarter of the final size. A test image was displayed.
GE also had a CdZnTe camera (see below). It is 3 mm thick now, working
on 5 mm thick, could be up to 10 mm thick. Interesting.

Digital Scintigraphics Inc., Waltham MA

Makers of Ceraspect dedicated brain scanner are planning to develop
a dedicated mammoscintigraphy camera. This a great idea. The folks who
brought you the first digital gamma camera, the first (and only) cylindrical
gamma camera are turning their creative minds to a superior way of imaging
the breast. New innovations include a small cylindrical shell of NaI with
a liquid light coupling medium. They should arrive at the optimum geometry
for mammoscintigraphy. Feasibility studies were very encouraging.

General Electric

GE also had a CZT camera in the booth. Definitely a works in progress.
They have an Israeli connection for the detectors. And developed with the
aid of your (US) tax dollars. That's leveraging your investments!

GE was also showing a mockup of a dual head camera "Millennium".
Very premature showing so it was hard to make firm judgments about it.
It was pretty. Will have big crystals, do whole bodies and turn out to
do sitting patients. Could not give specs on the coincidence detection
option.

Picked up a nice booklet: "Gamma Technology Briefing" Pub.
No. 96-4546 from GE. Is useful to understand what they are doing.

Harrison Medical Corp.

The Mountz Imaging System was shown by Harrison Medical Corp., Helena,
AL. It is a head fixing device based on eyeglass frames. Supposed to guarantee
reproducible head positioning across modalities. Full system includes software,
conversion software for different image formats.

MEDX

MEDX, Wood Dale, IL, continues to sell its Nuclear MAX, a PC based low
cost computer system. Also had a very interesting deluxe PC-based computer
system from an Argentine company named Sitco - NuQuest. This was a pretty
impressive machine, as demonstrated by its inventor and programming genius.

ONES Medical Services

There is a move to get independent service groups to come into a hospital
and bid against the manufacturers for service contracts. While this has
always been around to some extent, I have always cautioned against it,
especially for new systems. ONES Medical Services, Goffstown, NH is one
of these companies. They seem to be pretty successful at it. Also sell
the former Strichman computer, re-manufactured cameras, etc. We have been
having such poor service from GE I would be tempted.

Park Medical Systems Inc.

Park is not going to coincidence counting. They are re-evaluating coded
apertures. Using a modular coded aperture (random array of uniformly redundant
pinholes, modified with a blocking code), and taking advantage of <2
mm intrinsic camera resolution they hope to significantly improve sensitivity.
No scientific information is available yet. No trials till autumn. Interesting
but I am skeptical. They intend to do rotating tomograhy AND planar imaging
with this technology. First version is in W for low energy work, second
version will be in U for high energy work.

Their holospectral imaging remains unproven but interesting. No serious
trials or results have been presented. Independent evaluations of their
impressive technology would be desirable.

They have gone to 1/2"crystal for improved imaging at high energy.
Do not know if their depth correction technology is being used yet.

This is a company with lots of promise and lots of ideas. I think we
will have to just be patient to see results on these ideas.

Picker

Picker is in a slump. Can't find much new here. No viable coincidence
project. No STEP improvements released. Their EPICON detector project is
now part of the main company but it is not clear to the outside what is
happening. Maybe the bombshell will be heard in the future.

Siemens

This sleeping giant is stirring. A new camera, e.cam, looks pretty good.
All digital technology. Very flexible in positioning and clinical uses.
Will be developing coincidence technology, but nothing to report yet. Will
eventually have F-18 collimators. Will not be delivered until next year.
This camera will be able to receive the hybrid YSO/LSO detector that CTI
and Siemens are developing. That is a whole 'nother story. It is very unproven
and still being conceptually designed. Too early to put a confidence level
on it.

The e.cam will have a unique attenuation correction source, which will
be cheaper, and maybe more effective than some of those out there now.
Development seems to be in conjunction with a group in Vancouver. See No.
470 above. It uses two arrays of Gd-153 line sources of graded activity.
Have to watch this.

In computers they are moving to put all new software and port existing
software to run under IDL. IDL is software that runs on many different
platforms. Programs developed under IDL can also then run on many different
platforms. This sounds attractive. We will have to see if they can pull
it off.

General comment on cameras:

There is a lot of futures being sold right now. It is probably better
to focus on what will be delivered NOW. One important question that needs
to be addressed on these new cameras is collimator changing. It took ADAC
quite a while to get their automatic version working and now they are offering
a semi-automatic version. What are all those new cameras doing for collimator
changes? Collimators are getting heavier and they are multiplying.

Hope to see you at the 44th SNM meeting in San Antonio, Texas 1 June
- 5 June 1997